As a projection exposure apparatus in which a reticle pattern is projected and transferred on a silicon wafer, an apparatus using extreme ultraviolet (EUV) light having a wavelength of 13 to 14 nm as an exposure light source has been proposed. As the extreme ultraviolet light is greatly attenuated when it passes through an object, an optical control system using an optical lens cannot be employed. Accordingly, in the above exposure apparatus using the extreme ultraviolet light as an exposure light source, the exposure light is controlled by plural mirrors provided in a vacuum space (reflection optical system).
The reflection optical system of this exposure apparatus includes an exposure-light introduction optical system to guide exposure light from the light source to a reflective original plate (hereinbelow, reticle) and a reduced projection optical system for reduced projection of an exposure pattern on a wafer by the exposure light reflected from the reticle. The respective optical systems have plural mirrors. FIGS. 10A and 10B are explanatory views showing the form of a general concave-surface type mirror available in the above-described reflection optical system. The reflection surface of the mirror is a concave surface, however, a convex-surface type mirror can be employed. The mirror, having a reflection surface obtained by forming an Mo—Si multilayer film by vapor deposition or sputtering, reflects the exposure light from the light source.
However, upon reflection of exposure light by the above-described mirror, the reflectivity of the exposure light per one surface is about 70%, and the residual light is absorbed in the mirror base material and converted to heat. FIGS. 11A and 11B are explanatory views a temperature rise in the mirror. As shown in FIGS. 11A and 11B, in an exposure light reflection area, the temperature rises about +10 to 20° C. As a result, even if a mirror material having an extremely small thermal expansion coefficient is used, about 25 nm displacement occurs in the reflection surface in the exposure reflection area, and about 50 to 100 nm displacement occurs in the reflection surface in a mirror peripheral portion.
On the other hand, in a projection optical system mirror provided in the reduced projection optical system, an illumination system mirror and a light source mirror provided in the exposure-light introduction optical system, the accuracy of form of the reflection surface (hereinbelow, referred to as “accuracy of surface form”) must be 1 nm or smaller. Accordingly, as apparent from the above description, the extremely high accuracy of the surface form of about 1 nm cannot be ensured due to displacement of the mirror reflection surface by heat.
In the case of the projection optical system, the above degradation of the accuracy of the surface form in the mirror causes degradation of image formation performance and illumination on the wafer. For example, in the case of the illumination system mirror, the degradation of accuracy of the surface form causes a reduction of illumination and degradation of illumination evenness in the exposure light to a mask. Further, in the case of the optical source mirror, the degradation of the accuracy of the surface form causes degradation of illumination due to poor focusing of the light source. Such degradation causes degradation of basic performance of the exposure apparatus such as degradation of exposure accuracy and throughput.
Accordingly, it is desired to suppress a temperature rise of the mirror used in the reflection optical system of the exposure apparatus and to maintain the accuracy of the surface form of the mirror reflection surface.